1. Field of the Invention
The present invention relates to a continuous process for industrially preparing sodium percarbonate by the reaction of sodium carbonate with hydrogen peroxide.
2. Description of the Prior Art
Sodium percarbonate as well as sodium perborate has been used as a so-called oxygen-type bleaching agent. As compared with a so-called chlorine-type bleaching agent, sodium percarbonate is slightly inferior in the bleaching effect. However, sodium percarbonate acts more gently, for example, on cloth, thereby not damaging the same. Furthermore, sodium percarbonate does not produce a poisonous gas such as chlorine. Thus, sodium percarbonate as a bleaching agent has been recently increasingly used for home or business use.
As a final product, sodium percarbonate is usually produced in the granular form which is easy to be handled. For this purpose, crystalized sodium percarbonate is granulated with a granulator. The granulation can be more easily conducted and the granulated sodium percarbonate is more strengthened, by decreasing particle size of sodium percarbonate crystals. However, in case that sodium percarbonate is produced by crystallization, separation efficiency of the crystals from a slurry becomes worse by decreasing the particle size. On the other hand, by increasing the particle size, the loss of sodium percarbonate in powdery form increases upon granulation thereby to decrease the yield upon granulation, and the granulated sodium percarbonate is lowered in strength, too.
To increase strength of the granules, it has been proposed to add, upon granulation, an organic or inorganic binding agent to the crystals. As the organic binding agent, for example, a water-soluble high-molecular compound such as sodium polyacrylate, starch, or polyethylene glycol has been proposed. As the inorganic binding agent, for example, an aluminum compound such as aluminum phosphate or a silicate such as sodium silicate has been proposed. However, adding the binding agent upon granulation slows a so-called solution velocity of the granules in water upon practical use. The solution velocity is represented by the time required from the introduction of a product into a liquid to the complete dissolution of the same in the liquid. It takes more time to dissolve the granules by adding the binding agent upon granulation.
Thus, it is desired to maintain the particle size of sodium percarbonate crystals in a certain desired range to efficiently produce the granules having a satisfactory strength. Thus, it has been proposed to regulate their particle size by adjusting concentration of a slurry, intensity for stirring the slurry, the amount of a circulating mother liquor, or the like. However, the particle size is not satisfactorily regulated by the above-mentioned adjustments.
There are proposals, for example, JP-A-58-20815 and JP-A-58-217405, of regulating the particle size, in which fine seed crystals of sodium percarbonate pulverized by a dry or wet process are inoculated into a reaction vessel. However, upon pulverization with a grinder, the fine seed crystals tend to be contaminated with impurities such as iron and other metals, thereby accelerating decomposition of hydrogen peroxide and lowering the yield of sodium percarbonate crystals.
For practical use of sodium percarbonate granules as a bleaching agent, the granules are requested to be dissolved in water in a relatively short time. Thus, for example, JP-B2-56-21797 discloses sodium percarbonate granules mixed with a carbonate and an organic acid such as succinic acid, maleic acid or citric acid. Upon dissolution of the granules in water, carbon dioxide bubbles are generated in water by the reaction of the carbonate and the organic acid, thereby speeding up solution velocity of sodium percarbonate granules. However, oxygen content in a predetermined amount of sodium percarbonate granules, which oxygen is effective in bleaching, is lowered by increasing content of the organic acid therein. Furthermore, the organic acid is not suitable for some purpose.
To speed up solution velocity of the granules, the size of the same can be made small. However, this makes the same difficult to be handled.